The present invention relates to a valve train for a 4-cycle internal combustion engine of a so-called, "double overhead camshaft" or (DOHC) type, and, more particularly, to a valve driving device and a valve driving method for such internal combustion engine which can adjust the valve timing and the valve lift of an intake valve and an exhaust valve individually.
It is known that the valve timing and valve lift of an intake valve and an exhaust valve in a 4-cycle internal combustion engine have a large influence upon the performance of the engine. However, the optimum valve timing and the optimum valve lift will vary with a change in rotational speed of the engine. Therefore, if in the design of an engine an optimum valve timing and an optimum valve lift are selected for a certain rotational speed region, there occurs the problem that ideal performance cannot be obtained in the other rotational speed regions of the engine. To cope with this problem, there has been proposed, as described in Japanese Utility Model Publication No. 44-23442, a valve train capable of adjusting the valve timing and the valve lift of the intake valve and the exhaust valve according to a change in rotational speed of the internal combustion engine.
The valve train described in Japanese Utility Model Publication No. 44-23442 includes a camshaft having a cam contacting a rocker arm, a cam gear meshing an idler gear, and a camshaft supporting arm having one end pivotably supported with respect to a rotating shaft of the idler gear and the other end at which the camshaft is rotatably supported. The camshaft supporting arm is angularly displaceable about the rotating shaft of the idler gear according to a change in rotational speed of the internal combustion engine.
According to the above valve train, when the camshaft supporting arm is angularly displaced, the cam of the camshaft is moved along the slipper surface of the rocker arm toward or away from the fulcrum of the rocker arm, so that the leverage of the rocker arm is changed to thereby increase or decrease the valve lift of the exhaust valve and the intake valve. Simultaneously, the cam gear of the camshaft is rotated in mesh with the idler gear by the displacement of the camshaft supporting arm, so that the phase of the cam rotating together with the camshaft is changed to thereby change the valve timing of the exhaust valve and the intake valve.
However, as the above prior art valve train is adapted to the internal combustion engine of a so-called, "singe overhead camshaft" or (SOHC)-type wherein the intake valve and the exhaust valve are driven through a single camshaft, the valve timing and the valve lift of the intake valve and the exhaust valve cannot be adjusted individually, and it is accordingly difficult to sufficiently utilize the valve trains having this feature.
The present invention has been achieved in consideration of the above circumstances, and it is, accordingly, an object of the present invention to provide a valve driving device and a valve driving method for an internal combustion engine of a double overhead camshaft type in which the valve timing and the valve lift of the intake valve and the exhaust valve can be individually adjusted.